Railroad crossties have been made almost exclusively of wood from the beginning of the railroad age. The wooden crossties are held in place by ballast rocks and the rails are secured to the crossties by crosstie plates and spikes.
Because of the nature of timber, the wooden crossties accept and hold spikes, so that the rail and crosstie plate fastening systems can be secured to the crossties. This is the main advantage of the wooden crossties. In addition, a wood crosstie will flex under load. The resulting flexing is beneficial only in that it helps to provide for a softer ride. However, a wooden crosstie is susceptible to damages caused by exposure to sunlight, wind and rain. Thus, it easily deteriorates and ages, which shortens its service life. Therefore, it must be replaced regularly. This increases the cost of replacement and maintenance. Because forest resources are becoming scarce, it is necessary to search for alternative material instead of wood to make crossties.
To overcome the disadvantages of the wooden made crosstie, concrete crossties have been developed. However, conventional concrete crossties are too hard to use conventional and standard fastening systems (tie plates and spikes). Concrete crossties use pre-casted fasteners that are attached during the curing stage in the crosstie manufacturing process. Furthermore, each crosstie must be individually loaded and obstructed from the mold. At first glance, it would appear that the concrete crossties, since they are stiff and non-flexible, would be advantageous and provide a stiffer track module, improved lateral stability and gauge control, increased rail life, and greater locomotive fuel economy. However, what appeared to have been a significantly lower maintenance cost due to the lack of “pumping” of the ballast rock, has actually become another maintenance cost. The concrete crosstie is so hard that it pulverizes the ballast rock beneath it, which brings adversely effects the train operation. Thus, the ballast rocks need to be periodically replaced. More importantly, the concrete crossties provide a stiff and hard driving system without good shock absorption function, which limits the speeding up of trains.
Subsequently, railroad crossties made of material other than wood have been proposed. For example, U.S. Pat. No. 5,238,734 to Murray discloses a railroad crosstie made from a mixture of recycled tire fragments and an epoxy mixture. Other patents disclosing railroad crossties made out of composite materials include U.S. Pat. No. 4,150,790 (Potter) and U.S. Pat. No. 4,083,491 (Hill). Although crossties made out of composite materials provide significantly longer life than conventional wooden crossties, it has not been possible to provide composite crossties that are durable enough to withstand the heavy repeated loads of main line railroad tracks. Both wooden and composite railroad crossties tend to pump ballast rock away from the rails, thus requiring frequent reballasting.
A new type of composite crosstie disclosed by Chinese patent No. 99815337.0 was developed to maintain the advantages and to overcome those disadvantages of crossties such as wooden crosstie, concrete crosstie and the crosstie purely made of composite materials. The composite railroad crosstie is composed of two main parts: one part is an outer casing made of composite materials that are divided into an upper section and a lower section; the other part is a reinforcing inner core. The interface of the upper and the lower sections of the outer casing are attached together by urethane adhesive which is also used in aviation and is available from Mao Tao Corp. Then they are secured together by wooden screws, and special caps are glued on both ends of the crossties. The reinforcing inner core comprises three parts. The exterior part is a steel sheet shaped like “W” or “H” and stuffed with concrete in order to prevent concrete from being pulverized by long time impact of trains. The reinforcing inner core ensures the integral intensity of a crosstie. In order to fasten the rails to the crosstie by forcing spikes, inserts are placed into the core that are made out of the same composite material from which the casing is made, so that spikes can be driven through the casing, the apertures, and into the inserts.
On one hand, the high intensity and flexibility of the outer casing bring cushion and shock absorption to the heavy impact of trains. It may replace timber and it's more durable than timber. The materials of the outer casing are recycled plastic and recycled rubber that is not degrading. It can not only reduce environmental impact but also protect forest. On the other hand, rigidity of the concrete crosstie and the ballast rocks are so high that the ballast rocks get crushed by constant friction and vibration, which reduces the safety factor of railroad. Therefore, periodically manual maintenance used to be done. This results in further cost. In this aspect, the special material and structure of the outer casing of a composite crosstie make the ballast rock embeded in the outer cast by certain depth so as to reduce the relative movement between the crosstie and ballast rocks. It solves the problem of the concrete made crosstie.
However, the technical solution of the Chinese Patent No. 99815337.0 also has very clear disadvantages. It is illustrated in FIG. 7 that the outer casing 19 is composed of top and lower parts and there is a line between the two parts, which is the seam 19-1. An appropriate glue is applied to the interfaces of the two parts, and then the wooden screws 17 are used to fasten the two parts. The wooden screws are almost nothing to the force that the crosstie can bear. The wooden screws are very easy to be destroyed and the glue on the interfaces of the two parts is easy to be broken so that the top and the lower parts may mismatch or separate. Even a small mismatch would keep the two parts separate. Therefore, it increases the probability of destruction. Because there is certain space between the outer casing and the inner casing when the crosstie is subject to lateral force, such space exacerbates the abrasion and separation of the crosstie as a result of greatly reducing in reliability and service life and even causing the cracks and damage to the outer casing of the crosstie.
In addition, the manufacture techniques of the above composite crosstie have drawbacks. The production and assembly of the outer casing are separately done in two production lines, which results in extra investment on the equipment, more complicated procedure, more labor as well as a lower production rate.
The object of the invention is how to overcome the shortcomings as mentioned above.